Historical Development of Logic

Part I HISTORICAL DEVELOPMENT OF LOGIC 1 Ancient Greek Philosophical Logic ROBIN SMITH Ancient Greek logic was inseparable from ancient Greek philosophy. The formal theo- ries developed by major logicians such as Aristotle, Diodorus Cronus, and Chrysippus were in large part influenced by metaphysical and epistemological concerns. In this brief essay, I will try to give some picture of this interrelationship. For reasons of space, I make no attempt to cover, or even to mention, every aspect of ancient Greek logic. I have preferred instead to concentrate on illustrating its philosophical aspects. 1 The Origins: Parmenides and Zeno Greek philosophical logic originates with Parmenides (c. 510–c. 440 BCE). Though Parmenides cannot be said to have had a logic, or even an interest in studying the valid- ity of arguments, his views did much to set the agenda out of which many things in Greek philosophy, including logic, later arose. His philosophical position is both simple and mystifying: being is, whereas not being is not and cannot either be thought or said. Consequently, any type of expression that implies that being is not or that not being is must be dismissed as nonsense. For Parmenides, this includes any reference to change (since it must involve the coming to be of what is not and the not being of what is) or multiplicity (since to say that there are two things is to say that something is not something else). The conclusion is that what is is one, unchanging, and uniform, without distinctions. Much of subsequent Greek philosophy is an effort to avoid these consequences and defend the coherence of talk of motion and multiplicity. A second, and more explicitly logical, impact of Parmenides’ thought on Greek philosophy is through its defense by Parmenides’ follower Zeno of Elea (c. 490–c. 430 BCE). According to Plato’s Parmenides, Zeno’s goal was to defend Parmenides’ views from the objection that they were absurd or in contradiction to our ordinary beliefs. In response, Zeno argued that the beliefs that there is motion and that there is a multiplicity of enti- ties have consequences that are even more absurd because self-contradictory. This was the point of his celebrated arguments against motion and multiplicity. To consider one example, Zeno gives the following argument (paraphrased) that motion is impossible: 11 ROBIN SMITH In order to move from point A to point B, you must first reach the point halfway between them. But before you can reach that point, you must reach the point halfway to it. Continuing in this way, we see that before you can reach any point, you must already have reached an infinity of points, which is impossible. Therefore, motion is impossible. This argument rests only on the assumptions that motion is possible, that in order to move from one point to another one must first pass through the point halfway between, and that there is a point halfway between any two points. Zeno’s arguments take a particular form: beginning with premises accepted by his opponent, they derive conclusions that the opponent must recognize as impossible. Aristotle says that in introducing this form of argument, Zeno was the originator of ‘dialectic’. The meaning of this word is contested by scholars, but we may note three features of Zeno’s argument: (1) it is directed at someone else; (2) it takes its start from premises accepted by that other party; (3) its goal is the refutation of a view of that other party. These three characteristics can serve as a rough definition of a dialectical argument. 2 Dialectic and the Beginnings of Logical Theory In the later fifth century BCE, professional teachers of oratory appeared in Athens. These were most often the same people called (by us, by their contemporaries, and often by themselves) ‘Sophists’. We know that a number of the Sophists had interesting (and quite divergent) views on philosophical matters. Teaching oratory was a profitable occupation, and several Sophists seem to have amassed fortunes from it. The content of their instruction, to judge by later treatises on rhetoric, would have included such things as style and diction, but it would also have included some training in argumen- tation. That could have ranged from teaching set pieces of argument useful for specific situations, all the way to teaching some kind of method for devising arguments according to principles. One theme that emerges in several sophistic thinkers is a kind of relativism about truth. This is most forcefully put by Protagoras (c. 485–415 BCE), who began his treatise entitled Truth with the line, “Man is the measure of all things; of things that are, that they are, and of things that are not, that they are not.” Plato tells us in his Theaetetus that this meant “whatever seems to be true to anyone is true to that person”: he denied that there is any truth apart from the opinions of individuals. For Protagoras, this appears to have been connected with a thesis about the functioning of argument in a political situation. Whoever has the most skill at argument can make it seem (and thus be) to others however he wishes: in Protagoras’ world, persuasive speech creates not merely belief but also truth. Even apart from this perhaps extreme view, we find the themes of the variability of human opinion and the power of argument widespread in fifth-century Athens. Herodotus’ history of the Persian Wars present a picture of opinions about right and wrong as merely matters of custom by displaying the variability in customs from one people to another. The treatise known as the Twofold Arguments (Dissoi Logoi) gives a series of arguments for and against each of a group of propositions; the implication is that argument can equally well support any view and its contradictory. 12 ANCIENT GREEK PHILOSOPHICAL LOGIC Contemporary with the Sophists was Socrates (469–399 BCE), whose fellow Athenians probably regarded him as another Sophist. Socrates did not teach oratory (nor indeed does he appear to have taught anything for a fee). Instead, he engaged people he encountered in a distinctive type of argument: beginning by asking them questions about matters they claimed to have knowledge of, he would lead them, on the basis of their own answers to further questions, to conclusions they found absurd or to contradictions of their earlier admissions. This process, which Plato and Aristotle both saw as a form of dialectical argument, usually goes by the name of ‘Socratic refutation.’ In overall form, it exactly resembles Zeno’s arguments in support of Parmenides. Socrates insisted that he knew nothing himself and that his refutations were merely a tool for detecting ignorance in others. Plato (428/7–348/7 BCE) did not develop a logical theory in any significant sense. However, he did try to respond to some of the issues raised by Parmenides, Protagoras, and others. In his Theaetetus, he argues that Protagoras’ relativistic conception of truth is self-refuting in the sense that if Protagoras intends it to apply universally, then it must apply to opinions about Protagoras’ theory of truth itself; moreover, it implies that the same opinions are both true and false simultaneously. He also partially rejects Parmenides’ thesis that only what is can be thought or said by distinguishing a realm of ‘becoming’ that is not simply non-being but also cannot be said simply to be without qualification. Plato’s most celebrated philosophical doctrine, his theory of Forms or Ideas, can be seen as a theory of predication, that is, a theory of what it is for a thing to have a property or attribute. In very crude outline, Plato’s response is that what it is for x (e.g. Socrates) to be F (e.g. tall) is for x to stand in a certain relation (usually called ‘partici- pation’) to an entity, ‘the tall itself,’ which just is tall. In his Sophist, Plato begins to develop a semantic theory for predications. He observes that truth and falsehood are not properties of names standing alone but only of sentences produced by combining words. ‘Theaetetus’ and ‘is sitting’ are, in isolation, meaningful in some way but neither true nor false. We find truth or falsehood only in their combination: ‘Theaetetus is sitting.’ For Plato, a major achievement of this analysis is that it allows him to under- stand falsehoods as meaningful. In the sentence ‘Theaetetus is flying,’ both ‘Theaetetus’ and ‘is flying’ are meaningful; their combination is false, but it is still meaningful. Aristotle (384–322 BCE), Plato’s student, developed the first logical theory of which we know. He follows Plato in analyzing simple sentences into noun and verb, or subject and predicate, but he develops it in far greater detail and extends it to sentences which have general or universal (katholou, ‘of a whole’: the term seems to originate with Aristotle) subjects and predicates. Aristotle also gives an answer to Protagoras and to related positions. Specifically, in Book IV of his Metaphysics, he argues that there is a proposition which is in a way prior to every other truth: it is prior because it is a proposition which anyone who knows anything must accept and because it is impossible actually to disbelieve it. The proposition in question is what we usually call the principle of non-contradiction: “it is impossible for the same thing to be both affirmed and denied of the same thing at the same time and in the same way” (Met. IV.3, 1005b19–20). He argues that it follows from this principle itself that no one can disbelieve it.

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